I stumbled upon this beautiful photo and the photographer claimed to have waved a credit card instead of using a grad ND filter. Is this a known technique?
Well, it's just the same as "dodging" when making a print using an enlarger. If the exposure is long enough you could wave a card in front of the lens and reduce the maount of light from a selected part of the image.
However the exposure would need to be long or the card would show up and there's a good chance you might get some flare from exposure of the card itself.
In an emergency it's worth trying, but it can't replace a real ND grad filter (or an HDR image if you don't mind digital "trickery").
Yeah, I guess I will stick with the real filter for now. As far as HDR goes, I think it is a good technique to learn, but I would resist using it as much as I can.
There's no shame in using HDR when the scene calls for it. However I do think it's often used too much to produce cartoon like images. In the best HDR images you probably can't tell that HDR was used.
HDR is just another tool. It can be used well or it can be used badly.
I probably misworded myself, Bob. Just like you said, good HDRs are these used very subtly, just like the good photos using Grad ND filters (and thus I said it is a good technique). Also like you said the problem is there are too many in-your-face HDR products out there, and they become gimmicky quickly the same way if someone shoots exclusively using fish-eye lenses.